An electromagnetic wave absorber is a material that is designed to exhibit a balance between wave reflection, wave transmission and wave absorption or otherwise influence an electromagnetic wave incident upon it. The interaction between an electromagnetic wave and a medium is described completely by the complex permittivity and permeability. In the case of a non-magnetic medium, the complex permittivity describes the material completely, and thus the reflection, transmission and absorption coefficients. An efficient or effective electromagnetic wave absorber is one that minimises surface reflection and at the same time has sufficient absorptive properties so that transmitted radiation is reduced. The main object is to replace the appearance of an object by a smaller or different one determined by a cloaking material designed to hide the object.
In designing an electromagnetic wave absorber, one attempts to employ substances, which offer control of the loss mechanism and by way of this, offer control of the parameters governing the magnitude of the incident reflection. Sometimes other physical properties may play a role in the ability to influence the absorption or alteration of electromagnetic radiation. The thermal conductivity and emisivity are two parameters that can be exploited to further alter the appearance of a covered body.
In the present state of the art, control of microwave reflectivity has been demonstrated by simultaneous control of the bulk density of the material and the volume concentration of additives used to introduce the loss. The substances employed to introduce loss within the scope of the present state of the art are typically substances that exhibit Ohmic losses. At a sufficient volume fraction of this additive, a controlled interparticle contact between the Ohmic particles is achieved which produces macroscopic conductivity throughout the bulk of the medium. A proper balance between the macroscopic conductivity and density produce materials which can exhibit excellent absorptive properties over a wide band, typically between 2 to 18 GHz. This is but a rather narrow part of the entire microwave frequency band. The effective bandwidth is a result of employing an Ohmic loss mechanism in that Ohmic losses produce a hyperbolic frequency dependent loss factor. Thus, at low frequencies, the losses are so great that a degredation in surface reflection properties are produced while at high frequencies, the loss is so small that the material is not absorptive enough to prohibit high transmission and subsequent rereflection of an incident electromagnetic wave.
Typically, carbon powder or foamed forms of carbon or resistive sheets have been used and structures built from them produce excellent absorptive properties between 1 to 20 GHz in the microwave frequency band. In general, an electrically homogeneous material exhibiting a specific level of Ohmic conductivity can only produce good reflection loss over a narrow frequency band. Combinations of materials having different impedances may be used to covet wide parts of this band. Extremely thick shaped profiles are also used to produce broad-banded behaviour, especially at MHz frequencies.
Nature, however, offers another type of loss mechanism, dielectric relaxation. Dielectric relaxation is not an Ohmic process and is based on the fact that small molecules having a dipole moment totate in the presence of a modulating electromagnetic field. Theoretically, the process is described by the “Debye relaxation process”. The most common example of the use of dielectric relaxation in the absorption of microwaves is microwave drying and heating microwave heating and cooking is done in almost every household world wide. The size of the molecule and its dipole moment govern where maximum interaction with the field will occur and thus the frequency span of absorption of microwave energy and its transformation into thermal heating. Various physical limitations are associated with the exhibition of dielectric losses in materials.
Firstly, for rotation to occur, the molecules must be free to do so. This limits the material to liquids or gasses. The size of the molecule is associated with this in that size (inertial effects) requires that the molecule has a low inertia enabling it to rotate in phase to some extent with the electromagnetic radiation. Such small molecules are typically gasses and liquids as based on their melting or boiling point. Gasses ate typically too dilute to be of any use as a microwave absorber and are in any case hard to confine. Liquids, even though they are a condensed phase are typically too dense to be used as a microwave absorber. Most substances do exhibit some degree of dielectric telaxation, however, the absorption may not be as efficient as others.
Although the effect one is trying to achieve in microwave absorption is similar to that used in microwave heating or cooking, it should be realised that although many substances such as food stuffs absorb microwave energy, no food stuff or any natural substance in itself is designed by man to absorb microwave energy efficiently or maximally.
It has been known for quite some time that water, disposed in the form of an aerosol or fine droplets can attenuate microwave tadiation without producing a high initial reflection as water would in its dense state. Rain most certainly is not a stable structure as it is susceptible to gravity and wind, its density cannot be controlled widely and otherwise has to be continually generated.
It is an object of this invention to provide a novel type of electromagnetic energy adaptation material.